In a quantum communication system, information is sent between a transmitter and a receiver by encoded single quanta, such as single photons. Each photon carries one bit of information encoded upon a property of the photon, such as its polarization, phase, time or energy. The photon may even carry more than one bit of information, for example, by using properties such as angular momentum.
Quantum key distribution (QKD) is a technique that results in the sharing of cryptographic keys between two parties: a transmitter often referred to as “Alice”; and a receiver often referred to as “Bob”. The attraction of this technique is that it allows to quantify the maximum information potentially known to an unauthorised eavesdropper, often referred to as “Eve”. In many forms of QKD, Alice and Bob use two or more non-orthogonal bases in which to encode the bit values. These bases are kept secret during the quantum communication and are disclosed in a public discussion only after all the measurements have been completed by Bob. The laws of quantum mechanics dictate that measurement of the photons by Eve without prior knowledge of the encoding basis causes an unavoidable change to the quantum state of some of the photons.